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Smooth Muscle Antibody

Autoimmune hepatitis is an unresolving inflammation of the liver of unknown etiology that is associated with high concentrations of autoantibodies, pronounced hypergammaglobulinemia and the absence of cholestasis.  The definitive diagnosis requires the exclusion of viral, hereditary and drug induced liver disease. Autoimmune hepatitis occurs predominantly in women and affects all ages. It is responsible for up to 18% of chronic hepatitis cases not attributable to viruses or alcohol.

 Two types of autoimmune hepatitis have been proposed based on autoantibody associations.

Type Population Affected Autoantibody
1 Young to middle aged women ANA and/or ASMA

Western European children

Rare in U.S.

Liver kidney microsomal (LKM) antibody


Eighty percent of individuals with autoimmune hepatitis have type 1 AIH. Type 1 autoimmune liver disease is characterized by the presence of high levels of anti-nuclear antibody (ANA) or anti-smooth muscle antibody (ASMA). ASMA are IgG or IgM antibodies that are often directed against F-actin. Since F-actin is present in all smooth muscle fibers, these antibodies are not organ specific. The sensitivity and specificity of anti-actin antibodies for AIH are 80% and 90%, respectively. ANA and ASMA levels fluctuate during the disease course. Antibody levels cannot be used to predict the clinical course or outcome.

The presence of LKM autoantibodies has been used to define type 2 AIH, which is typically negative for ANA and ASMA. Type 2 AIH is more common in children and is often more aggressive than type 1 AIH. In the United States, only about 4% of adults with AIH have type 2 disease.

Traditionally, ASMA were detected with an immunofluorescent assay that looked for antibody binding to smooth muscle in tissue sections such as mouse stomach. Results were expressed as a titer, which was the highest dilution of patient sera giving a positive result.  A titer greater than 1:20 was considered positive. With the immunofluorescent assay, many weakly positive ASMA were observed.  An enzyme immunoassay using purified F-actin filaments has better specificity. The reference range is shown in the following table.

Interpretation Value
Negative 0-15 Units
Weak Positive 16–30 Units
Positive >30 Units


The numerical result is reported along with a negative or positive interpretation. Specimen requirement is one red top or SST tube of blood.


Zeman MV et al. Autoantibodies and Liver disease: Uses and Abuses. Can J Gastroenterol 2010;24:225-31.

Terziroli Beretta-Piccoli B, Mieli-Vergani G, Vergani D: Serology in autoimmune hepatitis: A clinical-practice approach. Eur J Intern Med. 2018;48:35-43

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